Sleep queues

Sleep queues hold the contexts of threads that have no work to do at a particular time. A thread is put to sleep either for a specified period of time or forever.

The administration class (ADM) of virtual processors runs the system timer and special utility threads. Virtual processors in this class are created and run automatically. No configuration parameters affect this class of virtual processors.

The ADM virtual processor wakes up threads that have slept for the specified time. A thread that runs in the ADM virtual processor checks on sleeping threads at one-second intervals. If a sleeping thread has slept for its specified time, the ADM virtual processor moves it into the appropriate ready queue. A thread that is sleeping for a specified time can also be explicitly awakened by another thread.

A thread that is sleeping forever is awakened when it has more work to do. For example, when a thread that is running on a CPU virtual processor must access a disk, it issues an I/O request, places itself in a sleep queue for the CPU virtual processor, and yields. When the I/O thread notifies the CPU virtual processor that the I/O is complete, the CPU virtual processor schedules the original thread to continue processing by moving it from the sleep queue to a ready queue. The following figure illustrates how the database server threads are queued to perform database I/O.
Figure 1. How database server threads are queued to perform database I/O
The paragraph that precedes this figure describes the content of the figure. In the figure, two threads are ready to continue processing when a third thread yields and three other partially executed threads are waiting for completion of their disk I/O requests.

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